Internal combustion engines convert the energy contained in a fuel into kinetic energy. An internal combustion engine has at least one combustion chamber in which the fuel is burned. The volume expansion generated during combustion is subsequently converted into rotational motion. To obtain an ignitable and efficient mixture for the combustion process, the fuel is mixed beforehand, with ambient air, in particular with the oxygen (O2) contained therein.
Historically, the desired fuel mixture was provided by a carburetor that was located outside the combustion chamber. Nowadays, injection systems are prevalent. In an injection system, the mixture formation now takes place almost exclusively within the combustion chamber. In direct injection, which is utilized here, a fuel injector injects precisely dosed quantities of fuel into the air-filled combustion chamber. The fuel atomizes when injected and is combined with the air in the combustion chamber. By utilizing this process, low emission, reliable combustion is achieved. Internal combustion engines may be divided into applied-ignition engines and auto-ignition engines. Otto-cycle engines are applied-ignition engines, whereas diesel engines are categorized as auto-ignition engines. In Otto-cycle engines, the mixture situated in the combustion chamber is first compressed and then ignited. This combustion is often accomplished by means of an ignition plug. In diesel engines, the air in the combustion chamber is compressed, rapidly increasing its temperature. The temperature generated in this process is sufficient to initiate auto-ignition of the diesel fuel that is subsequently injected into the compressed combustion chamber.
Besides using fuel in liquid form, such as gasoline, diesel, liquefied gas (Autogas, LPG) or liquefied natural gas (LNG), gas phase fuels are also used. Common examples of gas phase fuels in use are compressed natural gas (CNG) and hydrogen (H2). Further alternative fuels such as ethanol (C2H6O) or methanol (CH4O) may also be used.
Since the fuel injector must introduce the fuel directly into the combustion chamber, the fuel injector's tip is directly exposed to the heat generated during the combustion process. The injector tip is either partially located within the combustion chamber, or directly facing it. In this installation position, the injector tip must withstand not only the high combustion temperatures, but also temperature shocks and high injection pressures. The effects of corrosive combustion products formed must also be taken into consideration. These corrosive products play a significant role when using alternative fuels.
A fuel injector must maintain reliable operation independent of the driving cycle, driving performance of the vehicle, the respective climatic condition, and the fuel used. It is therefore commonplace for a rust-resistant austenitic steel to be used as a suitable material for the highly loaded fuel injector.
After a certain period of operation, some combustion products, such as soot or oil carbons, have been found deposited on the fuel injector. Such deposits form earlier and more readily on a passivation, protective layer composed of chromium oxide (Cr2O3) than on a surface composed of copper (Cu) or brass (CuZn).
Prior art suggests that the fuel injector should be coated with a suitable material in order to minimize or prevent these depositions from forming.
Accordingly, DE 199 51 014 A1 discloses a direct fuel injector which releases fuels, such as gasoline or diesel, into the combustion chamber of an internal combustion engine. The fuel injector has, for this purpose, an injector tip with at least one outlet opening for the fuel. A coating is suggested in order to maintain the spray parameters and prevent the tip from being adversely altered by the deposition of fuel and soot particles.
The coatings are formed from three groups of proposed materials, with each group having specific properties of interest. The first group consists of cobalt or nickel oxides, and oxides of alloys of the stated metals. These oxides are intended to prevent both the catalytic conversion (combustion) of soot particles that have already been deposited and the further deposition of carbon particles. This group also includes noble metals such as ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), platinum (Pt), and alloys of these metals with one another or with other metals. The second group of metals is intended to change the wetting behavior on the surface of the fuel injector so that the fuel droplets form beads and can be entrained by the surrounding flow. This second group includes ceramic coatings, metal-containing or metal-free carbon coatings, and fluorine-containing coatings or sapphire coatings. The third and final group is comprised of nitrite coatings, such as titanium nitride (TiN) or chromium nitride (CrN), and of oxide coatings (eg. tantalum oxide, TaO, or titanium oxide, TiO). This third group's purpose is to prevent the formation of a reaction layer on the fuel injector.
DE 42 22 137 B4 describes a fuel injection nozzle that can be used in diesel internal combustion engines. The fuel injector has a tip with at least one spray hole. In order to fabricate a spray hole with a cross sectional area that cannot be formed using conventional production methods, a coating is provided that extends into the spray hole. This reduces the effective cross section of the spray hole, which narrows in frustoconical form toward the outlet side. To apply the coating, the use of a hard material is proposed. For example, chromium (Cr), nickel (Ni), nickel-phosphorus, nickel-boron, nickel-cobalt-boron, aluminum oxide (Al2O3), chromium oxide (Cr2O3), titanium oxide (TiO2), chromium carbide (Cr3C2), silicon dioxide (SiO2), (AlSi), (NiCr), (WTi) or (WC) will suffice.
In order to prevent deposits from forming on a fuel injector for an internal combustion engine, JP 2007-309167 A proposes that the surface of the fuel injector be coated around an injection hole. Titanium oxide (TiO) is proposed as a photocatalytic coating.
JP 2005-155618 A discloses a method for the formation of a uniform coating, composed of titanium oxide (TiO), on an injection nozzle of a fuel injector for an internal combustion engine. This coating is applied in order to prevent, or at least reduce, any enrichment of carbon deposits. It is proposed that at least one section of the injection nozzle is initially dipped into a film-forming, undiluted solution of titanium ammonium fluoride ((NH4)2TiF6) and boric acid (H3BO3). Titanium oxide (TiO) is subsequently deposited on the surface of a valve seat and on the inner side of the injector tip in order to form a titanium oxide coating.
According to the cited teaching, the aim is to reduce the enrichment of deposits on the injector tips of fuel injectors. It is important to maintain the structural spray characteristics of the outlet openings for as long as possible. Hitherto, consideration has not been given to the emissions-related disadvantages that likewise arise from these soot deposits. With continuous use, uncontrolled combustion of fuel residue thickens the soot layer, and causes an increase in the emission of volatile organic substances (HC emissions). These residues are then ignited in an undesired manner by means of the fuel injector, whose lining covered tip continues to exhibit afterglow at the end of a combustion cycle, in a similar manner to a glow plug.
Even though it has already been possible to achieve a reduction of deposits by means of the coatings proposed in the prior art, the design of the fuel injector in question still offers room for improvement. In particular, it offers improvement with regard to the emissions generated by uncontrolled combustion events.